The surface characteristics of paper are important to both manufacturers and users of a broad range of paper products including printing grade paper, tissue, newsprint, linerboard, and so forth. For example, in the manufacture of paper and paperboard, printability is an important surface property to measure and control. Printability is not only important for printing grades of paper such as magazine paper and newsprint but also for linerboard and Kraft paper since, increasingly, the outside of boxes and bags are printed with manufacturers' logos and other information.
"Printability" refers to the characteristics of paper that make high quality printing possible. The properties affecting printability include, among others, surface smoothness and surface compressibility. Surface smoothness is considered the most important characteristic for all printing processes. High points on the surface tend not to hold ink because the pressure between the high points and the printing plate squeegees out the ink. Low points on the surface are unable to receive ink because they never contact the printing plate. With respect to surface compressibility, some printing processes such as rotogravure require a paper with a high degree of compressibility. The high pressure used in the printing press allows the surface to conform to the printing plate. Paper with a relatively rough surface in the uncompressed state can nevertheless have good printability qualities because of its surface compressibility.
Up to a point, increased smoothness, that is, decreased deviations from an ideal plane, enhances the printability of paper. A smooth surface tends to prevent ink from infiltrating the interstices of the paper fiber substrate in a fashion similar to the absorption of ink by an ink blotter. Thus, there is a significant relationship between the surface smoothness and print quality, that is, print density uniformity.
In the sense used herein, "smoothness" (and its complement, "roughness") refers to the microtopography of the surface of the sheet. Smoothness measurements are not concerned with the absolute location of the surface but rather with the extent to which the surface location deviates or varies from an ideal or mean plane. Such smoothness height variations are extremely small, being of the order of 10 micrometers. To obtain meaningful smoothness measurements, it is also necessary to know the scale size or interval (i.e., wavelength) over which the height variations occur. Small height variations occurring over a distance of several centimeters would have little effect on smoothness and consequently, printability, while the same variations occurring over only a 1 mm interval could have significant effects. For purposes of determining smoothness in the sense in which that term is used herein, it is most useful to determine height variations within several specific scale size ranges, for instance 20 to 100 micrometers, 100 to 200 micrometers, 200 to 400 micrometers, etc. Depending on the process, some scale size ranges will be more important than others.
Laboratory instruments have been developed which have achieved de facto standard status within the paper industry for the determination of surface smoothness. Because of their widespread popularity, it is desirable that any technique for determining paper smoothness on-line provides results that correlate well with the results produced by these recognized de facto standard laboratory instruments.
Traditionally, surface smoothness, as a predictor of printability, has been measured in the laboratory by various kinds of air leak tests, such as Sheffield, Parker Print Surf (PPS), BEKK and Bendtsen. The instruments used in these tests generally consist of a gas-confining wall or cylinder having an end surface placed in contact with the surface of a test sheet. Gas from a pressurized source is admitted into the cylinder and the rate at which the gas leaks past the interface of the cylinder end and paper surface is used as a determination of paper surface smoothness; obviously, the rougher the paper surface the faster the air escapes from the cylinder. The contacting surface of the air leak gauge may be a flat annular area or a knife edge and the leak rates will differ for these different contact surface geometries. Despite their popularity, air leak gauges tend not to work well with paper surfaces that are very smooth.
Recently, a new smoothness criterion called "Micro Average" was introduced by Emveco, Inc., Newberg, Oreg., U.S.A., and has found some industry acceptance for predicting printability of various paper products, particularly linerboard. Emveco manufactures a line of gauges for making surface profile measurements from which the "Micro Average" can be calculated. One such gauge uses a stylus having a radius of 0.00125 inch to measure the height of the sheet surface at a succession of points spaced along the test surface at equal intervals of, for example, 0.005 inch. As many as 500 readings or more are taken. The "Micro Average" is the average difference between successive readings over the entire set of readings.
Like the air leak testers, the Emveco gauge is a laboratory instrument that cannot be used on-line. Paper manufacturers, however, need a continuous indication of surface smoothness of the moving paper sheet as it is being produced. In this way, an immediate indication of printability is available, allowing the manufacturer to make corrections in the production process as needed in the event smoothness departs from a target value. Moreover, any such on-line measurements should correlate well with the results of standard laboratory tests.
Attempts have been made to satisfy the need for sensing smoothness on-line. In this connection, the prior art includes on-line, non-contact, optical surface roughness measuring apparatus. Most common are laser triangulation sensors in which a laser beam is focused on the surface to be measured. A lens focuses the image of the incident laser spot onto a position sensitive detector. The location of the image determines the location of the surface. The advantages of these prior art laser triangulation position sensors are their simplicity and accuracy. Among their disadvantages are, first, that the kind of position sensitive detectors typically used, CCDs and lateral cells, have limited frequency response and second, that as the surface position moves up and down (due to sheet "flutter", for example), the size of the spot changes since it goes out of focus; as a result, the scale size of the measured variations changes. For example, if the laser spot is focused to a diameter of 20 micrometers at the focal point and the surface moves 5 mm from the focal point the spot size will be 250 micrometers. A spot this large averages out the surface variations of interest thereby diminishing the usefulness of the sensor.
The prior art includes micro-focusing systems which attempt to solve the spot size variation problem with an automatic focusing device. The automatic focusing device moves the focusing objective lens to keep the spot focused on the surface to be measured. Micro-focusing systems can maintain a 1 micrometer spot size over a 1 mm range of up and down motion. The position of the lens is then measured to determine the position of the surface. The advantage of this system is that the spot size remains constant even when the surface moves up and down. The disadvantage of this device is that its speed is limited since the lens must be moved mechanically. Accordingly, microfocusing systems have frequency responses of only up to about 1200 Hz.
An example of a prior art on-line optical surface sensor is disclosed in U.S. Pat. No. 4,019,066 issued Apr. 19, 1977. As explained in that patent, the device illuminates the moving sheet, preferably at a low angle. Light scattered from the sheet is collected and processed by means of a photoelectric system. The electrical signals thus generated are divided into AC and DC components which are separately measured and their ratio is used as an index of roughness. Because this instrument senses the intensity of backscattered light and not spot position, it does not provide accurate results for smooth paper.
U.S. Pat. No. 4,092,068 issued May 30, 1978 discloses an on-line optical surface sensor in which, again, the intensity of light scattered from the surface of a traveling sheet is detected, in this case by two angularly spaced photodetector cells whose outputs suppress local reflectivity changes resulting from dirt or the like on the surface of the sheet. The incident light beam from an incandescent source is projected perpendicular to the surface of the sheet and illuminates a light spot having a relatively large diameter of 0.1 to 0.2 mm. This device, like that disclosed in U.S. Pat. No. 4,019,066, cannot provide accurate readings from smooth surfaces.
To measure sheet surface smoothness, attempts have also been made to use on-line gloss gauges which measure light reflected from paper. These approaches have likewise met with only partial success since paper, because of its surface properties, tends to provide both specular and non-specular (i.e., diffuse) reflections, with decreasing smoothness resulting in more diffuse reflections. Accordingly, there is often little relationship between gloss and smoothness.
Other examples of prior art non-contacting, optical systems for on-line measurement of the irregularities in the surfaces of moving sheets are disclosed in U.S. Pat. Nos. 4,102,578 and 5,110,212, and in a technical article by Schmidt, "Smoothness measurement in paper making and printing", published in Paper, 19 Apr. 1982, pages 24 et seq.
To our knowledge, none of the prior art on-line optical smoothness sensors completely satisfy all of the many requirements that must be met in order to be truly useful with today's papermaking machines. An acceptable on-line smoothness sensor must provide information that correlates well with that furnished by accepted laboratory smoothness testers, but must in addition be capable of providing accurate smoothness measurements for the smoothest printing grades. Because of the line speeds involved (which may exceed 1,200 meters per minute) and the need to resolve surface features having amplitudes and wavelengths as small as 0.1 micrometers and 20 micrometers, respectively, the incident light spot must be small, no more than about 20 micrometers in diameter, and the sensor must have an exceedingly high frequency response. The range of the sensor must be adequate to cover the complete range of the positions of the surface features of interest while at the same time preserving sensitivity to small positional variations. Because of the small amplitudes of the signals produced by the microtopography of the sheet surface, noise introduced by the sensor must be minimized in order to obtain a usable output signal. Moreover, the position of the moving sheet must be stabilized along the optical axis of the incident beam, that is, sheet "flutter" must be minimized in the region of the sensor so as to maintain the focus of the incident light spot and preserve measurement resolution. Still further, because of the vast quantity of information provided at the output of a smoothness sensor meeting the foregoing requirements, and because there are several types of printing processes each requiring different surface properties for best printing results, the information must be so processed that it can be displayed to the machine operator and used as a process control parameter in a meaningful and practical fashion. Last, provision must be made for automatic standardization of the sensor.
In sum, a need remains for an on-line instrument that will give immediate and accurate measurements, correlatable with standard laboratory test methods, of smoothness so as to be able to determine whether or not the paper being fabricated will be printable and that will provide such measurements for the smoothest printing grades. Further, paper product manufacturers need to be able to control the paper fabricating process so as to control the smoothness, and hence the printability, of the paper being made, and to do so in response to accurate on-line measurements of smoothness that are meaningful for the paper product being manufactured.
Certain macrotopographical surface features are also of interest to manufacturers of sheet material such as paper. For example, in the manufacture of tissue, creping is an important way of increasing the texture and softness of the tissue. Creping is the process of putting small folds in the tissue sheet. The depth and spacing of the folds imparts texture to the sheet. For example, tissue with large spacings between folds will feel coarser than a sheet with close spacing.
As far as can be determined, no on-line measurement of creping is currently available. Although laboratory testers exist for measuring the height and spacings of creping folds, the use of these testers is time consuming. The quality of tissue being produced is typically assessed in a subjective fashion, based on no more than the "feel" of the tissue and a visual examination thereof by an experienced operator. Based on these assessments, the flow rate of the spraybar applying adhesive to the Yankee cylinder may be adjusted and/or the creping or doctor blade which strips the tissue from the Yankee cylinder may be replaced. The doctor blades wear out quite fast, in some cases as often as once per hour. Thus, a technique for determining accurately when to change blades could result in significant savings to a mill. Changing blades too soon results in loss of production and increased blade costs. Changing blades too late results in the production of waste product.
Thus, there is a need for an on-line sensor for measuring tissue creping and using such a measurement for the control of the creping process and for the determination of blade wear and the need for blade replacement.